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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.extender"></a>Extender Manual</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="extender.html#bbv2.extender.intro">Introduction</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extender.example">Example: 1-to-1 generator</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extending.targets">Target types</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extending.tools">Tools and generators</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extending.features">Features</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extending.rules">Main target rules</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extending.toolset_modules">Toolset modules</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extender.intro"></a>Introduction</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="extender.html#bbv2.extender.overview.metatargets">Metatargets</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extender.overview.targets">Concrete targets</a></span></dt>
<dt><span class="section"><a href="extender.html#bbv2.extender.overview.generators">Generators</a></span></dt>
</dl></div>
<p>
        This section explains how to extend Boost.Build to accomodate your
        local requirements&#8212;primarily to add support for non-standard
        tools you have. Before we start, be sure you have read and understoon
        the concept of metatarget, <a class="xref" href="overview.html#bbv2.overview.concepts" title="Concepts">the section called &#8220;Concepts&#8221;</a>,
        which is critical to understanding the remaining material.
      </p>
<p>
        The current version of Boost.Build has three levels of targets, listed
        below.
      </p>
<div class="variablelist"><dl>
<dt><span class="term">metatarget</span></dt>
<dd><p>
              Object that is created from declarations in Jamfiles. May
              be called with a set of properties to produce concrete
              targets.
            </p></dd>
<dt><span class="term">concrete target</span></dt>
<dd><p>
              Object that corresponds to a file or an action.
            </p></dd>
<dt><span class="term">jam target</span></dt>
<dd><p>
              Low-level concrete target that is specific to Boost.Jam build
              engine. Essentially a string&#8212;most often a name of file.                
            </p></dd>
</dl></div>
<p>
        In most cases, you will only have to deal with concrete targets and
        the process that creates concrete targets from
        metatargets. Extending metatarget level is rarely required. The jam
        targets are typically only used inside the command line patterns.
      </p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>All of the Boost.Jam target-related builtin functions, like
        <code class="computeroutput">DEPENDS</code> or <code class="computeroutput">ALWAYS</code> operate on jam
        targets. Applying them to metatargets or concrete targets has no
        effect.</p></td></tr>
</table></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="bbv2.extender.overview.metatargets"></a>Metatargets</h4></div></div></div>
<p>Metatarget is an object that records information specified
        in Jamfile, such as metatarget kind, name, sources and properties,
        and can be called with specific properties to generate concrete
        targets. At the code level it is represented by an instance of
        class derived from <a class="link" href="reference.html#bbv2.reference.class.abstract-target" title="Class abstract-target">abstract-target</a>. 
        <sup>[<a name="idp213131072" href="#ftn.idp213131072" class="footnote">16</a>]</sup>
        </p>
<p>The <a class="link" href="reference.html#bbv2.reference.class.abstract-target.generate">generate</a>
        method takes the build properties
        (as an instance of the <a class="link" href="reference.html#bbv2.reference.class.property-set" title="Class property-set">
        property-set</a> class) and returns
        a list containing:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>As front element&#8212;Usage-requirements from this invocation
          (an instance of <a class="link" href="reference.html#bbv2.reference.class.property-set" title="Class property-set">
          property-set</a>)</p></li>
<li class="listitem"><p>As subsequent elements&#8212;created concrete targets (
          instances of the <code class="computeroutput">virtual-target</code> class.)</p></li>
</ul></div>
<p>It's possible to lookup a metataget by target-id using the 
        <code class="computeroutput">targets.resolve-reference</code> function, and the
        <code class="computeroutput">targets.generate-from-reference</code> function can both
        lookup and generate a metatarget.</p>
<p>The <a class="link" href="reference.html#bbv2.reference.class.abstract-target" title="Class abstract-target">abstract-target</a>
        class has three immediate derived classes:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><a class="link" href="reference.html#bbv2.reference.class.project-target" title="Class project-target">project-target</a> that
          corresponds to a project and is not intended for further
          subclassing. The <a class="link" href="reference.html#bbv2.reference.class.project-target.generate">
          generate</a> method of this
          class builds all targets in the project that are not marked as
          explicit.</p></li>
<li class="listitem"><p><a class="link" href="reference.html#bbv2.reference.class.main-target" title="Class main-target">main-target</a>
          corresponds to a target in a project
          and contains one or more target alternatives. This class also should not be 
          subclassed. The <a class="link" href="reference.html#bbv2.reference.class.main-target.generate">generate</a>
          method of this class selects an alternative to build, and calls the
          <a class="link" href="reference.html#bbv2.reference.class.basic-target.generate">generate</a>
          method of that alternative.</p></li>
<li class="listitem"><p><a class="link" href="reference.html#bbv2.reference.class.basic-target" title="Class basic-target">basic-target</a>
          corresponds to a specific target alternative. This is base class,
          with a number of derived classes.  The
          <a class="link" href="reference.html#bbv2.reference.class.basic-target.generate">generate</a> method
          processes the target requirements and requested build properties to
          determine final properties for the target, builds all sources, and
          finally calls the abstract
          <a class="link" href="reference.html#bbv2.reference.class.basic-target.construct">construct</a>
          method with the list of source virtual targets, and the final properties.
          </p></li>
</ul></div>
<p>The instances of the <a class="link" href="reference.html#bbv2.reference.class.project-target" title="Class project-target">project-target</a> and
        <a class="link" href="reference.html#bbv2.reference.class.main-target" title="Class main-target">main-target</a> classes are created
        implicitly&#8212;when loading a new Jamfiles, or when a new target
        alternative with as-yet unknown name is created.  The instances of the
        classes derived from <a class="link" href="reference.html#bbv2.reference.class.basic-target" title="Class basic-target">basic-target</a>
        are typically created when Jamfile calls a <em class="firstterm">metatarget rule</em>,
        such as such as <code class="computeroutput">exe</code>.
        </p>
<p>It it permissible to create a custom class derived from
        <a class="link" href="reference.html#bbv2.reference.class.basic-target" title="Class basic-target">basic-target</a> and create new metatarget rule
        that creates instance of such target. However, in the majority
        of cases, a specific subclass of <a class="link" href="reference.html#bbv2.reference.class.basic-target" title="Class basic-target">basic-target</a>&#8212;
        <a class="link" href="reference.html#bbv2.reference.class.typed-target" title="Class typed-target">typed-target</a> is used. That class is associated
        with a <em class="firstterm">type</em> and relays to <em class="firstterm">generators</em>
        to construct concrete targets of that type. This process will be explained below.
        When a new type is declared, a new metatarget rule is automatically defined.
        That rule creates new instance of type-target, associated with that type.                
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="bbv2.extender.overview.targets"></a>Concrete targets</h4></div></div></div>
<p>Concrete targets are represented by instance of classes derived
        from <code class="computeroutput">virtual-target</code>. The most commonly used
        subclass is <code class="computeroutput">file-target</code>. A file target is associated
        with an action that creates it&#8212; an instance of the <code class="computeroutput">action</code>
        class. The action, in turn, hold a list of source targets. It also holds the 
        <a class="link" href="reference.html#bbv2.reference.class.property-set" title="Class property-set">property-set</a>
        instance with the build properties that should be used for the action.</p>
<p>Here's an example of creating a target from another target, <code class="computeroutput">source</code></p>
<pre class="programlisting">
local a = [ new action $(source) : common.copy : $(property-set) ] ;
local t = [ new file-target $(name) : CPP : $(project) : $(a) ] ;
</pre>
<p>The first line creates an instance of the <code class="computeroutput">action&gt;</code> class.
        The first parameter is the list of sources. The second parameter is the name
        a jam-level <a class="link" href="overview.html#bbv2.overview.jam_language.actions">action</a>.
        The third parameter is the property-set applying to this action. The second line
        creates a target. We specifie a name, a type and a project. We also pass the
        action object created earlier.  If the action creates several targets, we can repeat
        the second line several times.</p>
<p>In some cases, code that creates concrete targets may be invoked more than
        once with the same properties. Returning to different instance of <code class="computeroutput">file-target</code>
        that correspond to the same file clearly will result in problems. Therefore, whenever
        returning targets you should pass them via the <code class="computeroutput">virtual-target.register</code>
        function, that will replace targets with previously created identical ones, as
        necessary.<sup>[<a name="idp213154048" href="#ftn.idp213154048" class="footnote">17</a>]</sup>
        Here are a couple of examples:
</p>
<pre class="programlisting">
return [ virtual-target.register $(t) ] ;
return [ sequence.transform virtual-target.register : $(targets) ] ;
</pre>
<p>
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="bbv2.extender.overview.generators"></a>Generators</h4></div></div></div>
<p>In theory, every kind of metatarget in Boost.Build (like <code class="computeroutput">exe</code>, 
        <code class="computeroutput">lib</code> or <code class="computeroutput">obj</code>) could be implemented
        by writing a new metatarget class that, independently of the other code, figures
        what files to produce and what commands to use. However, that would be rather inflexible.
        For example, adding support for a new compiler would require editing several metatargets.
        </p>
<p>In practice, most files have specific types, and most tools
        consume and produce files of specific type. To take advantage of this
        fact, Boost.Build defines concept of target type and
        <a class="indexterm" name="idp213157632"></a>
        <em class="firstterm">generators</em>, and has special metatarget class
        <a class="link" href="reference.html#bbv2.reference.class.typed-target" title="Class typed-target">typed-target</a>.  Target type is merely an
        identifier. It is associated with a set of file extensions that
        correspond to that type. Generator is an abstraction of a tool. It advertises
        the types it produces and, if called with a set of input target, tries to construct
        output targets of the advertised types. Finally,
        <a class="link" href="reference.html#bbv2.reference.class.typed-target" title="Class typed-target">typed-target</a>
        is associated with specific target type, and relays the generator (or generators)
        for that type.
        </p>
<p>A generator is an instance of a class derived from <code class="computeroutput">generator</code>.
        The <code class="computeroutput">generator</code> class itself is suitable for common cases.
        You can define derived classes for custom scenarios.</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extender.example"></a>Example: 1-to-1 generator</h3></div></div></div>
<p>Say you're writing an application that generates C++ code. If
      you ever did this, you know that it's not nice. Embedding large
      portions of C++ code in string literals is very awkward. A much
      better solution is:</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Write the template of the code to be generated, leaving
            placeholders at the points that will change
          </li>
<li class="listitem">
            Access the template in your application and replace
            placeholders with appropriate text.
          </li>
<li class="listitem">Write the result.</li>
</ol></div>
<p>It's quite easy to achieve. You write special verbatim files that are
      just C++, except that the very first line of the file contains the name of a
      variable that should be generated. A simple tool is created that takes a
      verbatim file and creates a cpp file with a single <code class="computeroutput">char*</code> variable
      whose name is taken from the first line of the verbatim file and whose value
      is the file's properly quoted content.</p>
<p>Let's see what Boost.Build can do.</p>
<p>First off, Boost.Build has no idea about "verbatim files". So, you must
      register a new target type. The following code does it:</p>
<pre class="programlisting">
import type ;
type.register VERBATIM : verbatim ;
</pre>
<p>The first parameter to <a class="link" href="reference.html#bbv2.reference.modules.type.register">type.register</a> gives
      the name of the declared type. By convention, it's uppercase. The second
      parameter is the suffix for files of this type. So, if Boost.Build sees
      <code class="filename">code.verbatim</code> in a list of sources, it knows that it's of
      type <code class="computeroutput">VERBATIM</code>.</p>
<p>Next, you tell Boost.Build that the verbatim files can be
      transformed into C++ files in one build step.  A
      <em class="firstterm">generator</em> is a template for a build step that
      transforms targets of one type (or set of types) into another.  Our
      generator will be called <code class="computeroutput">verbatim.inline-file</code>; it
      transforms <code class="computeroutput">VERBATIM</code> files into <code class="computeroutput">CPP</code> files:

</p>
<pre class="programlisting">
import generators ;
generators.register-standard verbatim.inline-file : VERBATIM : CPP ;
</pre>
<p>
  </p>
<p>Lastly, you have to inform Boost.Build about the shell
       commands used to make that transformation.  That's done with an
       <code class="computeroutput">actions</code> declaration.

</p>
<pre class="programlisting">
actions inline-file
{
    "./inline-file.py" $(&lt;) $(&gt;)
}
</pre>
<p>




</p>
<p>
    Now, we're ready to tie it all together. Put all the code above in file
    <code class="filename">verbatim.jam</code>, add <code class="computeroutput">import verbatim ;</code> to
    <code class="filename">Jamroot.jam</code>, and it's possible to write the following
    in your Jamfile:
  </p>
<pre class="programlisting">
exe codegen : codegen.cpp class_template.verbatim usage.verbatim ;
</pre>
<p>
    The listed verbatim files will be automatically converted into C++ source
    files, compiled and then linked to the codegen executable.
  </p>
<p>
    In subsequent sections, we will extend this example, and review all the
    mechanisms in detail. The complete code is available in the
    <code class="filename">example/customization</code> directory.
  </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extending.targets"></a>Target types</h3></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="extender.html#bbv2.extending.scanners">Scanners</a></span></dt></dl></div>
<p>The first thing we did in the <a class="link" href="extender.html#bbv2.extender.intro" title="Introduction">intruduction</a> was declaring a
      new target type:
</p>
<pre class="programlisting">
import type ;
type.register VERBATIM : verbatim ;
</pre>
<p>
        The type is the most important property of a target. Boost.Build can
        automatically generate necessary build actions only because you
        specify the desired type (using the different main target rules), and
        because Boost.Build can guess the type of sources from their
        extensions.
      </p>
<p>The first two parameters for the <code class="computeroutput">type.register</code> rule
        are the name of new type and the list of extensions associated with
        it. A file with an extension from the list will have the given target
        type. In the case where a target of the declared type is generated
        from other sources, the first specified extension will be used.
      </p>
<p>Sometimes you want to change the suffix used for generated targets
      depending on build properties, such as toolset. For example, some compiler
      uses extension <code class="literal">elf</code> for executable files. You can use the
      <code class="computeroutput">type.set-generated-target-suffix</code> rule:
</p>
<pre class="programlisting">
type.set-generated-target-suffix EXE : &lt;toolset&gt;elf : elf ;
</pre>
<p>
    </p>
<p>A new target type can be inherited from an existing one.
</p>
<pre class="programlisting">
type.register PLUGIN : : SHARED_LIB ;
</pre>
<p>
      The above code defines a new type derived from
      <code class="computeroutput">SHARED_LIB</code>. Initially, the new type inherits all the
      properties of the base type - in particular generators and suffix.
      Typically, you'll change the new type in some way. For example, using
      <code class="computeroutput">type.set-generated-target-suffix</code> you can set the suffix for
      the new type. Or you can write special a generator for the new type. For
      example, it can generate additional metainformation for the plugin.
      In either way, the <code class="computeroutput">PLUGIN</code> type can be used whenever
      <code class="computeroutput">SHARED_LIB</code> can. For example, you can directly link plugins
      to an application.
    </p>
<p>A type can be defined as "main", in which case Boost.Build will
      automatically declare a main target rule for building targets of that
      type. More details can be found <a class="link" href="extender.html#bbv2.extending.rules.main-type">later</a>.
    </p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="bbv2.extending.scanners"></a>Scanners</h4></div></div></div>
<p>
          Sometimes, a file can refer to other files via some include system. To
          make Boost.Build track dependencies between included files, you need
          to provide a scanner. The primary limitation is that only one scanner
          can be assigned to a target type.
        </p>
<p>First, we need to declare a new class for the scanner:
</p>
<pre class="programlisting">
class verbatim-scanner : common-scanner
{
    rule pattern ( )
    {
        return "//###include[ ]*\"([^\"]*)\"" ;
    }
}
</pre>
<p>
          All the complex logic is in the <code class="computeroutput">common-scanner</code>
          class, and you only need to override the method that returns
          the regular expression to be used for scanning. The
          parentheses in the regular expression indicate which part
          of the string is the name of the included file.  Only the
          first parenthesized group in the regular expression will be
          recognized; if you can't express everything you want that
          way, you can return multiple regular expressions, each of
          which contains a parenthesized group to be matched.
        </p>
<p>After that, we need to register our scanner class:
</p>
<pre class="programlisting">
scanner.register verbatim-scanner : include ;
</pre>
<p>
            The value of the second parameter, in this case
            <code class="computeroutput">include</code>, specifies the properties that contain the list
            of paths that should be searched for the included files.
         </p>
<p>Finally, we assign the new scanner to the <code class="computeroutput">VERBATIM</code>
        target type:
</p>
<pre class="programlisting">
type.set-scanner VERBATIM : verbatim-scanner ;
</pre>
<p>
          That's enough for scanning include dependencies.
        </p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extending.tools"></a>Tools and generators</h3></div></div></div>
<p>
        This section will describe how Boost.Build can be extended to support
        new tools.
      </p>
<p>For each additional tool, a Boost.Build object called generator
        must be created. That object has specific types of targets that it
        accepts and produces. Using that information, Boost.Build is able
        to automatically invoke the generator. For example, if you declare a
        generator that takes a target of the type <code class="literal">D</code> and
        produces a target of the type <code class="literal">OBJ</code>, when placing a
        file with extention <code class="literal">.d</code> in a list of sources will
        cause Boost.Build to invoke your generator, and then to link the
        resulting object file into an application. (Of course, this requires
        that you specify that the <code class="literal">.d</code> extension corresponds
        to the <code class="literal">D</code> type.)
      </p>
<p>Each generator should be an instance of a class derived from the
        <code class="computeroutput">generator</code> class. In the simplest case, you don't need to
        create a derived class, but simply create an instance of the
        <code class="computeroutput">generator</code> class. Let's review the example we've seen in the
        <a class="link" href="extender.html#bbv2.extender.intro" title="Introduction">introduction</a>.
        
</p>
<pre class="programlisting">
import generators ;
generators.register-standard verbatim.inline-file : VERBATIM : CPP ;
actions inline-file
{
    "./inline-file.py" $(&lt;) $(&gt;)
}
</pre>
<p>
      </p>
<p>We declare a standard generator, specifying its id, the source type
        and the target type. When invoked, the generator will create a target
        of type <code class="literal">CPP</code> with a source target of
        type <code class="literal">VERBATIM</code> as the only source. But what command
        will be used to actually generate the file? In bjam, actions are
        specified using named "actions" blocks and the name of the action
        block should be specified when creating targets. By convention,
        generators use the same name of the action block as their own id. So,
        in above example, the "inline-file" actions block will be used to
        convert the source into the target.
      </p>
<p>
        There are two primary kinds of generators: standard and composing,
        which are registered with the
        <code class="computeroutput">generators.register-standard</code> and the
        <code class="computeroutput">generators.register-composing</code> rules, respectively. For
        example:
</p>
<pre class="programlisting">
generators.register-standard verbatim.inline-file : VERBATIM : CPP ;
generators.register-composing mex.mex : CPP LIB : MEX ;
</pre>
<p>
        The first (standard) generator takes a <span class="emphasis"><em>single</em></span>
        source of type <code class="computeroutput">VERBATIM</code> and produces a result. The second
        (composing) generator takes any number of sources, which can have either
        the <code class="computeroutput">CPP</code> or the <code class="computeroutput">LIB</code> type. Composing generators
        are typically used for generating top-level target type. For example,
        the first generator invoked when building an <code class="computeroutput">exe</code> target is
        a composing generator corresponding to the proper linker.
      </p>
<p>You should also know about two specific functions for registering
        generators: <code class="computeroutput">generators.register-c-compiler</code> and
        <code class="computeroutput">generators.register-linker</code>. The first sets up header
        dependecy scanning for C files, and the seconds handles various
        complexities like searched libraries. For that reason, you should always
        use those functions when adding support for compilers and linkers.
      </p>
<p>(Need a note about UNIX)</p>
<h4>
<a name="idp213198720"></a>Custom generator classes</h4>
<p>The standard generators allows you to specify source and target
        types, an action, and a set of flags. If you need anything more complex,
        
        you need to create a new generator class with your own logic. Then,
        you have to create an instance of that class and register it. Here's
        an example how you can create your own generator class:
</p>
<pre class="programlisting">
class custom-generator : generator
{
    rule __init__ ( * : * )
    {
        generator.__init__ $(1) : $(2) : $(3) : $(4) : $(5) : $(6) : $(7) : $(8) : $(9) ;
    }

}

generators.register
  [ new custom-generator verbatim.inline-file : VERBATIM : CPP ] ;
</pre>
<p>
        This generator will work exactly like the
        <code class="computeroutput">verbatim.inline-file</code> generator we've defined above, but
        it's possible to customize the behaviour by overriding methods of the
        <code class="computeroutput">generator</code> class.
      </p>
<p>There are two methods of interest. The <code class="computeroutput">run</code> method is
        responsible for the overall process - it takes a number of source targets,
        converts them to the right types, and creates the result. The
        <code class="computeroutput">generated-targets</code> method is called when all sources are
        converted to the right types to actually create the result.
      </p>
<p>The <code class="computeroutput">generated-targets</code> method can be overridden when you
        want to add additional properties to the generated targets or use
        additional sources. For a real-life example, suppose you have a program
        analysis tool that should be given a name of executable and the list of
        all sources. Naturally, you don't want to list all source files
        manually. Here's how the <code class="computeroutput">generated-targets</code> method can find
        the list of sources automatically:
</p>
<pre class="programlisting">
class itrace-generator : generator {
....
    rule generated-targets ( sources + : property-set : project name ? )
    {
        local leaves ;
        local temp = [ virtual-target.traverse $(sources[1]) : : include-sources ] ;
        for local t in $(temp)
        {
            if ! [ $(t).action ]
            {
                leaves += $(t) ;
            }
        }
        return [ generator.generated-targets $(sources) $(leafs)
          : $(property-set) : $(project) $(name) ] ;
    }
}
generators.register [ new itrace-generator nm.itrace : EXE : ITRACE ] ;
</pre>
<p>
        The <code class="computeroutput">generated-targets</code> method will be called with a single
        source target of type <code class="literal">EXE</code>. The call to
        <code class="computeroutput">virtual-target.traverse</code> will return all targets the
        executable depends on, and we further find files that are not
        produced from anything. 
        The found targets are added to the sources.
      </p>
<p>The <code class="computeroutput">run</code> method can be overriden to completely
        customize the way the generator works. In particular, the conversion of
        sources to the desired types can be completely customized. Here's
        another real example. Tests for the Boost Python library usually
        consist of two parts: a Python program and a C++ file. The C++ file is
        compiled to Python extension that is loaded by the Python
        program. But in the likely case that both files have the same name,
        the created Python extension must be renamed. Otherwise, the Python
        program will import itself, not the extension. Here's how it can be
        done:
</p>
<pre class="programlisting">
rule run ( project name ? : property-set : sources * )
{
    local python ;
    for local s in $(sources)
    {
        if [ $(s).type ] = PY
        {
            python = $(s) ;
        }
    }
    
    local libs ;
    for local s in $(sources)
    {
        if [ type.is-derived [ $(s).type ] LIB ]
        {
            libs += $(s) ;
        }
    }

    local new-sources ;
    for local s in $(sources)
    {
        if [ type.is-derived [ $(s).type ] CPP ]
        {
            local name = [ $(s).name ] ;    # get the target's basename
            if $(name) = [ $(python).name ]
            {
                name = $(name)_ext ;        # rename the target
            }
            new-sources += [ generators.construct $(project) $(name) :
              PYTHON_EXTENSION : $(property-set) : $(s) $(libs) ] ;
        }
    }

    result = [ construct-result $(python) $(new-sources) : $(project) $(name)
                 : $(property-set) ] ;
}
</pre>
<p>
        

        First, we separate all source into python files, libraries and C++
        sources. For each C++ source we create a separate Python extension by
        calling <code class="computeroutput">generators.construct</code> and passing the C++ source
        and the libraries. At this point, we also change the extension's name,
        if necessary.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extending.features"></a>Features</h3></div></div></div>
<p>
        Often, we need to control the options passed the invoked tools. This
        is done with features. Consider an example:
</p>
<pre class="programlisting">
# Declare a new free feature
import feature : feature ;
feature verbatim-options : : free ;

# Cause the value of the 'verbatim-options' feature to be
# available as 'OPTIONS' variable inside verbatim.inline-file
import toolset : flags ;
flags verbatim.inline-file OPTIONS &lt;verbatim-options&gt; ;

# Use the "OPTIONS" variable
actions inline-file
{
    "./inline-file.py" $(OPTIONS) $(&lt;) $(&gt;)
}
</pre>
<p>
        We first define a new feature. Then, the <code class="computeroutput">flags</code> invocation
        says that whenever verbatin.inline-file action is run, the value of
        the <code class="computeroutput">verbatim-options</code> feature will be added to the
        <code class="computeroutput">OPTIONS</code> variable, and can be used inside the action body.
        You'd need to consult online help (--help) to find all the features of
        the <code class="computeroutput">toolset.flags</code> rule.
        
      </p>
<p>
      Although you can define any set of features and interpret their values
      in any way, Boost.Build suggests the following coding standard for
      designing features.
    </p>
<p>Most features should have a fixed set of values that is portable
      (tool neutral) across the class of tools they are designed to work
      with. The user does not have to adjust the values for a exact tool.  For
      example, <code class="computeroutput">&lt;optimization&gt;speed</code> has the same meaning for
      all C++ compilers and the user does not have to worry about the exact
      options passed to the compiler's command line.
    </p>
<p>
      Besides such portable features there are special 'raw' features that
      allow the user to pass any value to the command line parameters for a
      particular tool, if so desired. For example, the
      <code class="computeroutput">&lt;cxxflags&gt;</code> feature allows you to pass any command line
      options to a C++ compiler. The <code class="computeroutput">&lt;include&gt;</code> feature
      allows you to pass any string preceded by <code class="computeroutput">-I</code> and the interpretation
      is tool-specific.  (See <a class="xref" href="faq.html#bbv2.faq.external" title="Can I get capture external program output using a Boost.Jam variable?">the section called &#8220;
      Can I get capture external program output using a Boost.Jam variable?
    &#8221;</a> for an example of very smart usage of that
      feature).  Of course one should always strive to use portable
      features, but these are still be provided as a backdoor just to make
      sure Boost.Build does not take away any control from the user.
    </p>
<p>
      Using portable features is a good idea because:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>When a portable feature is given a fixed set of
          values, you can build your project with two different
          settings of the feature and Boost.Build will automatically
          use two different directories for generated files.
          Boost.Build does not try to separate targets built with
          different raw options.
            
          </p></li>
<li class="listitem"><p>Unlike with &#8220;raw&#8221; features, you don't need to use
          specific command-line flags in your Jamfile, and it will be
          more likely to work with other tools.
          </p></li>
</ul></div>
<p>
    </p>
<h4>
<a name="idp213215552"></a>Steps for adding a feauture</h4>
<p>Adding a feature requires three steps:

        </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<p>Declaring a feature. For that, the "feature.feature"
              rule is used. You have to decide on the set of <a class="link" href="reference.html#bbv2.reference.features.attributes" title="Feature Attributes">feature
              attributes</a>:

              </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>if you want a feature value set for one target
                to automaticaly propagate to its dependant targets then make it
                &#8220;propagated&#8221;. </p></li>
<li class="listitem"><p>if a feature does not have a fixed list of
                values, it must be &#8220;free.&#8221;  For example, the <code class="computeroutput">include
                </code> feature is a free feature.</p></li>
<li class="listitem"><p>if a feature is used to refer to a path relative
                to the Jamfile, it must be a &#8220;path&#8221; feature. Such features will
                also get their values automatically converted to Boost Build's
                internal path representation. For example, <code class="computeroutput">include</code>
                is a path feature.</p></li>
<li class="listitem"><p>if feature is used to refer to some target, it
                must be a &#8220;dependency&#8221; feature. </p></li>
</ul></div>
<p>
              </p>
</li>
<li class="listitem"><p>Representing the feature value in a
          target-specific variable. Build actions are command
          templates modified by Boost.Jam variable expansions.  The
          <code class="computeroutput">toolset.flags</code> rule sets a target-specific
          variable to the value of a feature.</p></li>
<li class="listitem"><p>Using the variable. The variable set in step 2 can
              be used in a build action to form command parameters or
              files.</p></li>
</ol></div>
<p>
      </p>
<h4>
<a name="idp213221520"></a>Another example</h4>
<p>Here's another example.
        Let's see how we can make a feature that refers to a target. For example,
        when linking dynamic libraries on Windows, one sometimes needs to
        specify a "DEF file", telling what functions should be exported. It
        would be nice to use this file like this:
</p>
<pre class="programlisting">
        lib a : a.cpp : &lt;def-file&gt;a.def ;
</pre>
<p>

        Actually, this feature is already supported, but anyway...
        
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<p>Since the feature refers to a target, it must be "dependency".
</p>
<pre class="programlisting">
feature def-file : : free dependency ;
</pre>
<p>
            </p>
</li>
<li class="listitem">
<p>One of the toolsets that cares about
        
        DEF files is msvc. The following line should be added to it.
        

</p>
<pre class="programlisting">
flags msvc.link DEF_FILE &lt;def-file&gt; ;
</pre>
<p>
            
            </p>
</li>
<li class="listitem">
<p>Since the DEF_FILE variable is not used by the
msvc.link action,

we need to modify it to be:

</p>
<pre class="programlisting">
actions link bind DEF_FILE
{
    $(.LD) .... /DEF:$(DEF_FILE) ....
}
</pre>
<p>
            </p>
<p> Note the <code class="computeroutput">bind DEF_FILE</code> part. It tells
          bjam to translate the internal target name in
          <code class="varname">DEF_FILE</code> to a corresponding filename in
          the <code class="computeroutput">link</code> action.  Without it the expansion of
          <code class="computeroutput">$(DEF_FILE)</code> would be a strange symbol that is
          not likely to make sense for the linker.
          </p>
<p>
            We are almost done, but we should stop for a small workaround. Add the following
            code to msvc.jam

</p>
<pre class="programlisting">
rule link
{
    DEPENDS $(&lt;) : [ on $(&lt;) return $(DEF_FILE) ] ;
}
</pre>
<p>


            This is needed to accomodate some bug in bjam, which hopefully
            will be fixed one day.
            
</p>
</li>
</ol></div>
<h4>
<a name="idp213228160"></a>Variants and composite features.</h4>
<p>Sometimes you want to create a shortcut for some set of
        features. For example, <code class="computeroutput">release</code> is a value of
        <code class="computeroutput">&lt;variant&gt;</code> and is a shortcut for a set of features.
      </p>
<p>It is possible to define your own build variants. For example:
</p>
<pre class="programlisting">
variant crazy : &lt;optimization&gt;speed &lt;inlining&gt;off
                &lt;debug-symbols&gt;on &lt;profiling&gt;on ;
</pre>
<p>
        will define a new variant with the specified set of properties. You
        can also extend an existing variant:
</p>
<pre class="programlisting">
variant super_release : release : &lt;define&gt;USE_ASM ;
</pre>
<p>
        In this case, <code class="computeroutput">super_release</code> will expand to all properties
        specified by <code class="computeroutput">release</code>, and the additional one you've specified.
      </p>
<p>You are not restricted to using the <code class="computeroutput">variant</code> feature
      only.
      
      Here's example that defines a brand new feature:
</p>
<pre class="programlisting">
feature parallelism : mpi fake none : composite link-incompatible ;
feature.compose &lt;parallelism&gt;mpi : &lt;library&gt;/mpi//mpi/&lt;parallelism&gt;none ;
feature.compose &lt;parallelism&gt;fake : &lt;library&gt;/mpi//fake/&lt;parallelism&gt;none ;
</pre>
<p>

        This will allow you to specify the value of feature
        <code class="computeroutput">parallelism</code>, which will expand to link to the necessary
        library.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extending.rules"></a>Main target rules</h3></div></div></div>
<p>
      A main target rule (e.g &#8220;<a class="link" href="tasks.html#bbv2.tasks.programs" title="Programs">exe</a>&#8221;
      Or &#8220;<a class="link" href="tasks.html#bbv2.tasks.libraries" title="Libraries">lib</a>&#8221;) creates a top-level target. It's quite likely that you'll want to declare your own and
      there are two ways to do that.
      
    </p>
<p><a name="bbv2.extending.rules.main-type"></a>The first way applies when

      your target rule should just produce a target of specific type. In that case, a
      rule is already defined for you! When you define a new type, Boost.Build
      automatically defines a corresponding rule. The name of the rule is
      obtained from the name of the type, by downcasing all letters and
      replacing underscores with dashes.
      
      For example, if you create a module
      <code class="filename">obfuscate.jam</code> containing:

</p>
<pre class="programlisting">
import type ;
type.register OBFUSCATED_CPP  : ocpp ;

import generators ;
generators.register-standard obfuscate.file : CPP : OBFUSCATED_CPP ;
</pre>
<p>
      and import that module, you'll be able to use the rule "obfuscated-cpp"
      in Jamfiles, which will convert source to the OBFUSCATED_CPP type.
    </p>
<p>
      The second way is to write a wrapper rule that calls any of the existing
      rules. For example, suppose you have only one library per directory and
      want all cpp files in the directory to be compiled into that library. You
      can achieve this effect using:
</p>
<pre class="programlisting">
lib codegen : [ glob *.cpp ] ;
</pre>
<p>
      If you want to make it even simpler, you could add the following
      definition to the <code class="filename">Jamroot.jam</code> file:
</p>
<pre class="programlisting">
rule glib ( name : extra-sources * : requirements * )
{
    lib $(name) : [ glob *.cpp ] $(extra-sources) : $(requirements) ;
}
</pre>
<p>
      allowing you to reduce the Jamfile to just
</p>
<pre class="programlisting">
glib codegen ;
</pre>
<p>
    </p>
<p>
      Note that because you can associate a custom generator with a target type,
      the logic of building can be rather complicated. For example, the
      <code class="computeroutput">boostbook</code> module declares a target type
      <code class="computeroutput">BOOSTBOOK_MAIN</code> and a custom generator for that type. You can
      use that as example if your main target rule is non-trivial.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.extending.toolset_modules"></a>Toolset modules</h3></div></div></div>
<p>
      If your extensions will be used only on one project, they can be placed in
      a separate <code class="filename">.jam</code> file and imported by your
      <code class="filename">Jamroot.jam</code>. If the extensions will be used on many
      projects, users will thank you for a finishing touch.
    </p>
<p>The <code class="computeroutput">using</code> rule provides a standard mechanism
    for loading and configuring extensions.  To make it work, your module
    
    should provide an <code class="computeroutput">init</code> rule. The rule will be called
    with the same parameters that were passed to the
    <code class="computeroutput">using</code> rule. The set of allowed parameters is
    determined by you. For example, you can allow the user to specify
    paths, tool versions, and other options.
    
    </p>
<p>Here are some guidelines that help to make Boost.Build more
      consistent:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>The <code class="computeroutput">init</code> rule should never fail. Even if
          the user provided an incorrect path, you should emit a warning and go
          on. Configuration may be shared between different machines, and
          wrong values on one machine can be OK on another.
          
          </p></li>
<li class="listitem">
<p>Prefer specifying the command to be executed
        to specifying the tool's installation path. First of all, this
        gives more control: it's possible to specify
</p>
<pre class="programlisting">
/usr/bin/g++-snapshot
time g++

</pre>
<p>
            as the command. Second, while some tools have a logical
            "installation root", it's better if the user doesn't have to remember whether
            a specific tool requires a full command or a path.
            
          </p>
</li>
<li class="listitem">
<p>Check for multiple initialization. A user can try to
            initialize the module several times. You need to check for this
            and decide what to do. Typically, unless you support several
            versions of a tool, duplicate initialization is a user error.
            
            If the
            tool's version can be specified during initialization, make sure the
            version is either always specified, or never specified (in which
            case the tool is initialied only once). For example, if you allow:
</p>
<pre class="programlisting">
using yfc ;
using yfc : 3.3 ;
using yfc : 3.4 ;
</pre>
<p>
            Then it's not clear if the first initialization corresponds to
            version 3.3 of the tool, version 3.4 of the tool, or some other
            version. This can lead to building twice with the same version.
            
            </p>
</li>
<li class="listitem">
<p>If possible, <code class="computeroutput">init</code> must be callable
          with no parameters. In which case, it should try to autodetect all
          the necessary information, for example, by looking for a tool in
          <code class="envar">PATH</code> or in common installation locations. Often this
          is possible and allows the user to simply write:
</p>
<pre class="programlisting">
using yfc ;
</pre>
<p>
          </p>
</li>
<li class="listitem"><p>Consider using facilities in the
          <code class="computeroutput">tools/common</code> module. You can take a look at how
          <code class="computeroutput">tools/gcc.jam</code> uses that module in the <code class="computeroutput">init</code> rule.
          </p></li>
</ul></div>
<p>
    </p>
</div>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.idp213131072" href="#idp213131072" class="para">16</a>] </sup>This name is historic, and will be eventuall changed to 
        <code class="computeroutput">metatarget</code></p></div>
<div class="footnote"><p><sup>[<a id="ftn.idp213154048" href="#idp213154048" class="para">17</a>] </sup>This create-then-register pattern is caused by limitations 
        of the Boost.Jam language. Python port is likely to never create duplicate targets.</p></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2006-2009 Vladimir Prus<p>Distributed under the Boost Software License, Version 1.0.
      (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 
      <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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